1. Field of Invention
This invention pertains to a method and module for separating a composite gas into separate permeates with different levels of concentrations.
2. Description of the Prior Art
It is well known to use hollow monofilaments or fibers in various procedures for separating fluids and particularly gases due to their relative permeabilities through the hollow fiber substance. Typically, the fibers are formed into a cylindrical bundle to provide a large surface or contact area. The bundle is encased in a pressure shell providing an assembly which is commonly referred to as a module. Pressurized gas is fed into the shell and some of the gas permeates through the walls of the fibers and is bled off from the module as a permeate. The remaining gas is also bled off from the module as a residue or raffinate. The compositions of the permeate and raffinate are different from each other and from that of the feed mixture due to the selectivity of the hollow fiber wall substance. The fraction of the feed which is bled off as permeate is called the stage cut.
It has been found that while a single module may separate gases effectively at low stage cuts, the efficiency of separation declines with increasing stage cut. Therefore various schemes have been devised for processing the original feed and consequent residue gases of modules by staging two or more modules in series. Each module is characterized by permeate and residue gases of different concentrations and compositions. Obviously such arrangements require several modules which take time and effort and are costly to manufacture, and additionally they must be interconnected by piping.